function plane = NormalPlane(axis)
  % -----------------------------------------------------------------------
  % NormalPlane - Return the name of the coordinate plane normal to the
  %               given axis.
  %
  % Args: axis - 'x', 'y' or 'z': the axis
  %
  % Rtns: plane - 'yz', 'zx', or 'xy': the plane normal to the axis
  % -----------------------------------------------------------------------

  switch axis
    case 'x', plane = 'yz';
    case 'y', plane = 'zx';
    case 'z', plane = 'xy';
    otherwise, error('"%s" is not a valid axis', axis);
  end
end
